I flash magisk from sideload, to flash from twrp Ive read that you must change slot or flash twrp on both slots.
Pd: stock version seems pretty well, only a few LOS functions are better. Restore wasn't as good as stock (maybe Android q feature?)
Pd: I got a weird audio bug with whatsapp playing from close speaker.
it might be that when you pull down data from a previous backup made with a newer OS version, some incompatibilities rise and so Google's backup & restore operations don't fill in all the cracks like they do on the same version. I wouldn't be surprised if a lot of the newer stuff came with Q though.
as for the audio bug, is it "crackling"? that was fixed fairly recently in a Q update, and may not have been backported to P.